1. Field of the Invention
The present invention relates to a fuel cell power supply device for controlling an amount of reactive gases supplied to a fuel cell depending on an electric energy requested by a load.
2. Description of the Related Art
There have heretofore been employed fuel cell power supply devices which use fuel cells as a power supply device for vehicles such as electric vehicles. An amount of reactive gases (a reducing gas such as hydrogen and an oxidizing gas for extracting electrons by reacting with the reducing gas) to be consumed varies depending on an amount of an output current of the fuel cell. Therefore, it is necessary to control a flow rate of reactive gases so that the amount of reactive gases supplied to the fuel cell is not excessive or insufficient with respect to an electric energy requested by a load such as an electric motor.
One conventional fuel cell power supply device incorporates a control arrangement shown in FIG. 5 for determining a target output current (Ifc_CMD) for a fuel cell on a vehicle. First, a fuel cell vehicle control unit 100 calculates a requested electric energy (PD_CAL) of an electric motor 101 depending on an amount of depression (Ap) of an accelerator pedal and a vehicle speed (Nm).
An output limiting unit 102 calculates a target output (PD_REQ) to limit an upper limit of the requested electric energy (PD_CAL) depending on a capacity of a fuel cell 103 to generate an electric energy. A torque command calculator 104 calculates a torque command (TRQ_CMD) depending on the target output (PD_REQ) and then outputs the torque command to a motor driving unit 105.
On the other hand, a P/I converter 106 applies the target output (PD_REQ) to map data 107, stored in a memory, representing output current/voltage characteristics (I-V characteristics) of the fuel cell 103 to determine the target output current (Ifc_CMD) of the fuel cell 103. A reactive gas supply unit 108 supplies the reactive gases depending on the target output current (Ifc_CMD) to the fuel cell 103 so that the fuel cell 103 outputs a current appropriate to the target output (PD_REQ).
By the above processing, a driving electric energy needed to obtain the target output (PD_REQ) is supplied from the motor driving unit 105 to the electric motor 101, and the reactive gases needed to obtain the target output current (Ifc_CMD) depending on the target output (PD_REQ) are supplied from the reactive gas supply unit 108 to the fuel cell 103.
However, in some cases, the I-V characteristics of the fuel cell 103 vary (shown by a line B in FIG. 5) from an initial state (shown by a line A in FIG. 5) depending on a change in temperature, supply pressure, and humidity of the reactive gases and a change of the fuel cell 103 with time. When the I-V characteristics of the fuel cell 103 vary, the target output current (Ifc_CMD) set for the target output (PD_REQ) is improper, so that the current is excessively or insufficiently outputted from the fuel cell 103.
Therefore, the following processing is performed: A current (Ifc) and a voltage (Vfc) actually outputted from the fuel cell 103 are detected, a difference (ΔI) between the actual current (Ifc) and the target output current (Ifc_CMD) and a difference (ΔV) between the actual voltage (Vfc) and a target output voltage (Vfc_CMD) corresponding to the target output current (Ifc_CMD) in the I-V characteristics are calculated, and the I-V characteristics are corrected depending on the differences (ΔI, ΔV).
However, in the correction of the I-V characteristics as mentioned above, for example, as in the case of abrupt acceleration of the fuel cell vehicle, when a power of the electric motor 101 increases so as to exceed a response speed of the reactive gas supply unit 108 and the actual current (Ifc) and the actual voltage (Vfc) are insufficient with respect to the target output current (Ifc_CMD) and the target output voltage (Vfc_CMD) (in this case, the insufficient current is backed up by discharging an electric energy from a capacitor 109), the calculated differences (ΔI, ΔV) are larger than differences based on the actual initial values of the I-V characteristics of the fuel cell due to a delayed response from the reactive gas supply unit 108.
Accordingly, there are the following disadvantages: The I-V characteristics of the fuel cell 103 are corrected so as to be larger than the actual I-V characteristics. When the target output (PD_REQ) is applied to the corrected I-V characteristics, the target output current (Ifc_CMD) cannot be determined accurately.